Barclay was first elected to the State Assembly on November 5, 2002.[2] He won the November 2008 general election with 67 percent of the vote[4][5] and ran uncontested in the November 2010 and 2012 general elections.[6][7][8]

Barclay is currently the Deputy Minority Leader in the New York State Assembly,[9] ranker on the Insurance Committee and Chairman of the Republican Assembly Campaign Committee.

On December 14, 2007, William Barclay announced his candidacy for New York State Senator, in New York's 48th Senate district. He was seeking to replace former Senator Jim Wright, who announced his resignation on December 12, 2007. He lost the election to Democratic assemblyman Darrel Aubertine (who was defeated the following term by Patty Ritchie, a Republican).